Set Content-Length for Metrics requests

changelog.d/7730.bugfix

@@ -0,0 +1 @@
+Fix missing `Content-Length` on HTTP responses from the metrics handler.

synapse/metrics/_exposition.py

@@ -208,6 +208,7 @@ def do_GET(self):
             raise
         self.send_response(200)
         self.send_header("Content-Type", CONTENT_TYPE_LATEST)
+        self.send_header("Content-Length", str(len(output)))
         self.end_headers()
         self.wfile.write(output)
 
@@ -261,4 +262,6 @@ def __init__(self, registry=REGISTRY):
 
     def render_GET(self, request):
         request.setHeader(b"Content-Type", CONTENT_TYPE_LATEST.encode("ascii"))
-        return generate_latest(self.registry)
+        response = generate_latest(self.registry)
+        request.setHeader(b"Content-Length", str(len(response)))
+        return response